Differential amplifier and oscillator mixer comprising said amplifier

ABSTRACT

A differential amplifier of the cascode type comprises a differential stage which includes a first (T 1 ) and a second (T 2 ) transistor whose collectors are connected to an active load constituted by the respective collector-emitter paths of a third (T 3 ) and a fourth (T 4 ) transistor in series with a first and a second resistor, respectively. One end of each resistor is connected to a supply voltage terminal. The third and the fourth transistor have their bases connected to a reference voltage terminal. A first capacitor (C 1 ) is coupled between the collector-emitter paths of the third and of the first transistor, and a second capacitor (C 2 ) is arranged between the collector-emitter paths of the fourth and of the second transistor. A third (R&#39; 1 ) and a fourth (R&#39; 2 ) resistor are connected between the supply voltage terminal (V cc ) and the collector-emitter path of the first transistor and of the second transistor, respectively.

BACKGROUND OF THE INVENTION

This invention relates to a differential amplifier of the cascode type comprising a differential stage which comprises a first and a second transistor whose collector-emitter paths are connected to an active load and whose bases are connected to inputs of the differential amplifier, the active load comprising a third and a fourth transistor whose collector-emitter paths are connected to a first supply voltage terminal via a first resistor and a second resistor respectively and whose bases are connected to a reference voltage terminal.

Such amplifiers are widely known and used. A problem with these amplifiers is the relationship between linearity and supply voltage. For a given supply voltage it may be desirable to improve the linearity or, conversely, for a given linearity it may be desirable to reduce the supply voltage. Applicant's French Patent 2,640,094, filed on 6 Dec. 1988, proposes a solution to this problem.

SUMMARY OF THE INVENTION

It is an object of the invention to improve the operation of amplifiers of the above type.

The basic idea is to provide a different amplifier operation for direct current and for alternating current.

Thus, a differential amplifier in accordance with the invention is characterised in that it comprises a first capacitor, arranged between the collector-emitter paths of the third transistor and of the first transistor, and a second capacitor, arranged between the collector-emitter paths of the fourth transistor and of the second transistor, in that it comprises a third resistor and a fourth resistor, arranged between the first supply voltage terminal and the collector-emitter path of the first transistor and of the second transistor, respectively, and in that it comprises a first and a second current source connected to the collector-emitter path of the third transistor and the fourth transistor respectively to supply a current to the third transistor and to the fourth transistor. Said current sources may comprises a fifth resistor and a sixth resistor connected between the collector-emitter path of the third transistor and of the fourth transistor, respectively, and a second supply voltage source (for example, ground).

The invention also relates to an oscillator-mixer comprising a mixer driving the inputs of a differential amplifier as defined above, and having its outputs connected to a filter. Suitably, the mixer is coupled directly to the inputs of the differential amplifier. In a preferred embodiment the mixer is coupled to the outputs of the differential amplifier only via a first and a second feedback resistor.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in more detail, by way of non-limitative example, with reference to the accompanying drawings, in which:

FIG. 1 shows an oscillator-mixer in accordance with said French Patent 2,640,094,

FIG. 2 shows a differential amplifier in accordance with the invention,

FIG. 3 shows an oscillator-mixer in accordance with the invention, and

FIG. 4 shows a preferred embodiment of an oscillator-mixer in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 an amplifier suitable for use as an IF amplifier for the signals from a mixer (MIX, R₀, C₀, R'₀, C'₀) comprises two transistors T₁ and T₂ forming a differential pair, the emitters of the transistors being connected by two resistors R₁₁ and R₂₂ in series, and a current source (specifically a resistor R₃₃), the current source being connected to the node common to R₁₁ and R₂₂. The cascode transistors T₃ and T₄, whose bases are interconnected and are brought to the reference potential V_(ref1), have their collector-emitter paths arranged between the collector-emitter paths of T₁ and T₂, respectively and a supply voltage terminal V_(cc). Current sources (R₁₃, R₁₄) separately supply a part of the steady-state current of the differential pair (T₁, T₂).

Transistors T₅ and T₆ are arranged as emitter-followers, their bases being connected to the collectors of the transistors T₃ and T₄ respectively. The collectors of T₅ and T₆ are connected to the supply voltage terminal V_(cc). The emitters of T₅ and T₆ are connected to a current source (R₁₀, T₁₀) and R₂₀, T₂₀) respectively, for which purpose the transistors T₁₀ and T₂₀ have their bases connected to a reference voltage terminal V_(ref2). In order to obtain a transimpedance amplifier feedback is provided by means of two resistive divider bridges (R₅₁, R₅₂) and (R₆₁, R₆₂). The first divider bridge (R₅₁, R₅₂) is arranged between the emitter of T₅ and a second supply voltage terminal (common-mode terminal). The second divider bridge (R₆₁, R₆₂) is arranged between the emitter of T₆ and the second supply voltage terminal.

The filter of the amplifier is constituted by a parallel circuit L₁₀₀, C₁₀₀ R₁₀₀ connected to the emitters of T₅ and T₆ via two resistors R₁₅ and R₂₅ and two capacitors C₁₁ and C₁₂.

In FIG. 2 an amplifier in accordance with the invention, in the same way as that in FIG. 1, comprises a differential pair (T₁, T₂, R₁₁, R₂₂, I₃₃ (R₃₃)) and cascode transistors T₃ and T₄ whose bases are interconnected and are brought to the voltage V_(ref1). The collectors of T₃ and T₄ are connected to the supply voltage terminal Vcc via resistors R₁ and R₂. The collectors of T₁ and T₂ are connected to the supply voltage terminal Vcc via resistors R'₁ and R'₂.

Current sources (I₁₃ (R₁₃), I₁₄ (R₁₄)) are arranged in the emitters of T₃ and T₄ respectively to supply a current to T₃ and T₄. It is to be noted that the currents are directed oppositely to those supplied by the resistors R₁₃ and R₁₄ in the prior-art amplifier. The currents are bias currents whose values are non-critical.

The collector-emitter paths of T₃ and T₄ are connected in series with those of the transistors T₁ and T₂ via capacitors C₁ and C₂ respectively.

When the impedances of the capacitors C₁ and C₂ are sufficiently low the Miller effect of the transistors T₁ and T₂ decreases in the same way as in a conventional cascode amplifier.

On the other hand, as a result of the presence of the capacitors C₁ and C₂, in the case of direct current operation, the collector of T₁ (or of T₂) will be at a higher potential than the emitter of T₃ (or of T₄). In other words, if all the other conditions are the same, the supply voltage V_(cc) can be smaller than in a conventional cascode arrangement, in which allowance is to be made for the two collector-emitter paths in series. In accordance with the invention the transistors of the differential pair and the cascode transistors may be said to operate "in series" for alternating current and "in parallel" for the d.c. bias.

If the supply voltage V_(cc) remains the same, the values of the resistors R'₁ and R'₂ can be reduced in comparison with the prior art (because the collectors of T₁ and T₂ can be at a higher potential than before). This results in the transistors T₃ and T₄ being biassed in such a way that collector-emitter voltages are higher than before and, consequently, an improved linearity can be obtained. Moreover, R₃₃ may be replaced by a current source I₃₃, making it possible to achieve a substantially perfect rejection of common-mode signals.

Finally, the amplifier has a different d.c. and a.c. gain.

The d.c. gain G_(DC) is ##EQU1##

The a.c. gain G_(AC) is ##EQU2##

Since the value of R'₁ and of R'₂ has been reduced advantageously as indicated above, it is easy to obtain a distinctly smaller value for G_(DC) than for G_(AC), which makes it possible to dispense with the feedback of the amplifier for direct current, the offset voltages at the input being no longer annoying.

In FIG. 3 the current sources I'₁₃ and I'₁₄ comprise resistors R'₁₃ and R'₁₄ connected between the emitters of T₃ and T₄ respectively and ground. It is to be noted that the emitter voltage of T₃ and T₄ is comparatively constant because it only differs from V_(ref1) by the base-emitter voltage of T₃ and T₄, which varies little as a function of the current through the transistor.

In the same way as in FIG. 1, there are provided transistors T₅ and T₆, resistors R₁₅ and R₂₅, and current sources (R₁₀, T₁₀) and (R₂₀, T₂₀). The d.c. feedback resistors (R₅₁, R₅₂) and (R₆₁, R₆₂) have been dispensed with for the reasons already mentioned. The mixer MIX is coupled directly to the bases of the transistors T₁ and T₂. This is possible owing to the above-mentioned reduction of the value of R'₁ and R'₂, which enables the offset voltages at the amplifier input to be reduced, as a result of which a d.c. feedback is not necessary. The mixer is coupled to the outputs of the amplifier (emitters of the transistors T₅ and T₆) by a resistor in series with a capacitor, (R, C) for T₅ and (R', C') for T₆. In general, C and C' can be of the order of 10 pF, which means that the capacitors can be integrated (see European Patent Application EP 414,328, which corresponds to U.S. Pat. No. 5,072,194).

FIG. 4 shows a direct coupling. It is desirable to dispense with the capacitors C and C' for the mere reason of the area they occupy in an integrated circuit. Another reason is the risk of oscillation at very low frequency when the filter (R₁₀₀, L₁₀₀, C₁₀₀) is not connected. In FIG. 4 the coupling is provided directly by feedback resistors R and R' between the mixer and the outputs of the amplifier. To achieve this, the voltage drop is compensated for by increasing the currents of the current sources (R₁₀, T₁₀) and (R₂₀, T₂₀) by a value ΔI equal to ##EQU3## V_(A) being the potential on the base of the transistor T₁ for direct current, and V_(D) -V_(A) being the voltage drop across the feedback resistor (R or R').

It is also advisable to reduce the values of the resistors R₀ and R'₀ to maintain the voltages equal: ##EQU4## R_(M) being the new value of the resistor R₀, respectively the resistor R₀ ', and I_(M) being the current through the resistor R₀, respectively the resistor R₀ ' in the case of FIG. 3. 

I claim:
 1. A differential amplifier of the cascode type comprising: a differential stage which comprises a first and a second transistor whose collector-emitter paths are connected to an active load and whose bases are connected to inputs of the differential amplifier, the active load comprising a third and a fourth transistor whose collector-emitter paths are connected to a first supply voltage terminal via a first resistor and a second resistor, respectively, and whose bases are connected to a reference voltage terminal, characterised in that a first capacitor is coupled between the collector-emitter paths of the third transistor and of the first transistor, and a second capacitor is coupled between the collector-emitter paths of the fourth transistor and of the second transistor, a third resistor and a fourth resistor coupled between the first supply voltage terminal and the collector-emitter path of the first transistor and of the second transistor, respectively, and a first and a second current source connected to the collector-emitter path of the third transistor and the fourth transistor, respectively, to supply a current to the third transistor and to the fourth transistor.
 2. A differential amplifier as claimed in claim 1, wherein the first and second current source comprise a fifth and a sixth resistor connected between the collector emitter path of the third transistor and of the fourth transistor respectively, and a second supply voltage terminal.
 3. An oscillator-mixer comprising a mixer driving the inputs of a differential amplifier having its outputs connected to a filter, wherein the differential amplifier includes an amplifier as claimed in claim
 2. 4. An oscillator-mixer comprising a mixer driving the inputs of a differential amplifier having its outputs connected to a filter, wherein the differential amplifier includes an amplifier as claimed in claim
 1. 5. An oscillator-mixer as claimed in claim 4, wherein the mixer is coupled directly to the input of the differential amplifier.
 6. An oscillator-mixer as claimed in claim 5, wherein the mixer is coupled to outputs of the differential amplifier only via a first and a second feedback resistor.
 7. An oscillator-mixer as claimed in claim 6, wherein a fifth transistor and a sixth transistor connected as emitter followers have their bases connected to a first point and a second point, respectively, the first point being connected between the collector-emitter path of the third transistor and the first resistor, and the second point being connected between the collector-emitter path of the fourth transistor and the second resistor.
 8. An oscillator-mixer as claimed in claim 4, wherein a fifth transistor and a sixth transistor connected as emitter followers and have their bases connected to a first point and a second point, respectively, the first point being connected between the collector-emitter path of the third transistor and the first resistor, and the second point being connected between the collector-emitter path of the fourth transistor and the second resistor. 